1. Field of the Invention
This invention generally relates to a pallet exchange apparatus for a machine tool. More particularly, it relates to a pallet exchange apparatus for automatically exchanging a pallet in an upright posture relative to a machine tool table.
2. Description of the Related Art
A machine tool often requires a pallet on which a work is set securely for machining with a machine tool. Where the machine tool uses such a pallet, the pallet carrying a work is fixed on the table of the machine tool before machining and then removed from the table after machining in order to replace it with another pallet with a new work.
In view of productivity and labor-saving in machining a large number of works, a pallet-utilizing machine tool is associated with a pallet exchange apparatus which provides an automatic pallet exchange function.
Typically, a conventional pallet exchange apparatus comprises a rotary arm which is rotatable in a vertical plane about a horizontal spindle, two pallet receivers each provided at a respective end of the rotary arm for removably receiving a pallet in the vertical plane of the rotary arm, and a setup stand for holding the pallet in an upright posture and transferring it onto a selected one of the pallet receivers. The pallet carries a work (normally made of a metal) which projects laterally from the vertical plane of the rotary arm when received by the relevant pallet receiver.
In operation of the conventional pallet exchange apparatus, the rotary arm first turns to a position where one of the pallet receivers (referred to as "first pallet receiver" just for convenience of description) is close to the machine tool table while the other pallet receiver (referred to as "second pallet receiver") is close to the setup stand. In this position, the first pallet receiver receives an old pallet (because it carries a finished work) from the machine tool table, whereas the second pallet receiver receives a new pallet (because it carries a work which requires machining) from the setup stand. Then, the rotary arm turns by 180 degrees to bring the first pallet receiver close to the setup stand for replacement with another pallet at the setup stand while bringing the second pallet receiver close to the machine tool table for fixing the new pallet thereto.
In this way, the conventional pallet exchange apparatus provides an automatic pallet exchanging function for enhancing the productivity. However, the conventional pallet exchange apparatus still has the following disadvantages.
As described above, the work held by each pallet receiver projects laterally from the vertical plane of the rotary arm. Thus, the weight of the work acts to twist the rotary arm, which may cause a positional deviation of the pallet relative to the setup stand and/or the machine tool table to make automatic mounting of the pallet difficult. This problem is particularly remarkable when the weight of the work is large.